kraftwerk

Hiya, just my two cents worth here.

In all cases, first do a full backup. If you don't have a tape/disk backup regime, make sure that the stuff on the drives showing up in Explorer are cloned/imaged somewhere.

If only one of the two original drives is faulty in a RAID1, replacing the drive and going into the RAID controller software should enable you to rebuild.

Regarding the HDD sizes, if the old drive is 18GB and the replacement is 73GB, all you lose is drive space, since RAID will only use 18GB of the new drive. However, if you get two 73GB drives, you should be able to a) restore the initial RAID1, then b) clone/image the contents as before, swap the remaining 18GB with a new 73GB and c) let the controller rebuild the container. Then it should be possible to use the unused 55-ish GB to create a new container and eventually populate that with new data.

I'm not familiar with the Adaptec 2100, but considering an older Adaptec 29160 is only around US$70, and a new Adaptec 39160 is about US$130 on eBay, it might be useful to update the RAID controller as well.

With the higher-end Adaptec cards, a RAID5 or RAID 5+1 configuration would be a more feasible (and safer/faster) configuration for the future.